Christopher Fred Keimel

Strain dependent facet stabilization in selective-area heteroepitaxial growth of GaN nanostructures

We report on the selective-area heteroepitaxy and facet evolution of submicron GaN islands on GaN-sapphire, AlN-sapphire, and bare sapphire substrates. It is shown that strain due to the lattice mismatch between GaN and the underlying substrate has a significant influence on the final morphology and faceting of submicron islands. Under identical metalorganic chemical vapor deposition growth parameters, islands with low or no mismatch strain exhibit pyramidal morphologies, while highly strained islands evolve into prismatic shapes. Furthermore, islands grown with relatively low compressive mismatch strain yield more uniform arrays of pyramids as compared to the nonstrained, homoepitaxially grown crystals. It is proposed that the strain dependency of Ehrlich-Schwoebel barriers across different crystallographic planes could potentially account for the observed morphologies during selective area growth of GaN islands.

Power switch system based on Microelectromechanical switch

A power switching system combining a MEMS (Microelectromechanical systems) switch and an arc suppression circuit has been developed. The system, using a single 3mm × 3mm MEMS switch die coupled with protection electronics, has been demonstrated to switch both resistive loads and inductive loads to a peak power of around 800W. The entire opening and closing operations are completed in a few microseconds, orders of magnitude faster than mechanical relays and nearly as fast as solid state devices. The MEMS switch is an array of microscale cantilevered relays that are electrostatically opened and closed in microseconds. A pulsed diode bridge protection circuitry is used in conjunction with the MEMS switch to open and close the energized contacts without damaging the contacting surface and without generating an arc. This MEMS based power switching system is scaled in both series and parallel to increase the switched voltage and current capability respectively. MEMS switch based power switching has the capability to enable both current limiting and arc free switching, characteristics that can significantly reduce dangerous downstream fault energy for AC and DC protection and distribution applications.